1. Field of the Invention
The present invention relates to a stainproof protector for preventing a mask for lithography from staining. More particularly, it relates to a stainproof protector so-called pellicle, which is useful for preventing a photo mask or reticule from deposition of dust during the lithographic process for the production of semiconductors.
2. Discussion of the Background
Lithography is widely used for forming a desired pattern on a substrate in the field of the production of semiconductors. In the production of semiconductors, a reticule or photo mask is used as the mask for lithography. However, if a foreign matter such as dust is deposited on this mask, the shade of this foreign matter is likely to be transferred to the substrate, thus leading to a defective product. Further, when such a mask is to be used again, it is necessary to clean the mask, and the cleaning is likely to bring about defects such as decrease in the image density, disappearance of an image line, falling off of the image line edge, formation of pinholes, etc. Therefore, it is common to employ a stainproof protector having a structure such that a protective film is supported by a ring-shaped frame such as a circular or rectangular frame, to prevent the mask from staining. It is advantageous to use such a stainproof protector not only for protecting the mask from staining but also in that when an enlarged mask such as a reticule is employed, even if some foreign matters are deposited on the protective film, formation of their images on the substrate can be avoided by displacing the position of the protective layer out of the focal length, and thus it is possible to prevent formation of defective products. As the protective film for such a stainproof protector, it has been common to employ a thin film made of nitorcellulose or cellulose acetate. Such a thin film is usually employed in the form of a multi layer film having a so-called reflection preventive coating layer applied in order to obtain a high transmittance for light having a wave length useful for lithography.
However, in order to prepare such a reflection preventive coating film, a relatively expensive production method such as vacuum vapor deposition is required, and there has been a problem that cracking or peeling of the reflection preventive coating film is likely.
Further, Japanese Unexamined Patent Publication No. 83032/1985 proposes to use polymers of fluoroolefins such as tetrafluoroethylene, vinylidene fluoride and hexafluoroethylene for protective films. However, with these polymers, light scattering is likely to take place in a low wavelength region because of their crystallinity, and non-crystalline polymers are poor in creep resistance and have a problem that the protective films are likely to undergo deflection.
Further, in recent years, there has been a trend that for the purpose of improving the degree of integration of semiconductors, light in an ultraviolet region having a short wave length is used in the process of lithography. However, the conventional protective film has a problem that the ultraviolet resistance is poor, and no adequate durability is obtainable in such use.